Among methods of fabricating polycrystalline diamond by the direct conversion from graphite, a method described in Patent Literature 1 has been known. This method can fabricate polycrystalline cubic diamond directly from graphite at an ultrahigh pressure and high temperature without adding a sintering aid and/or a catalyser. The polycrystalline diamond has an average crystalline size of 100 nm or less, a purity of at least 99% and a hardness of at least 110 GPa; these are properties that make the polycrystalline diamond industrially usable for a tool tip or the like.
In general, materials referred to as diamond, which include a material described in Patent Literature 1, have a cubic crystal structure. On the other hand, it has been known that there is also a hexagonal polymorph called hexagonal diamond or lonsdaleite, natural mineral name. Hexagonal diamond is composed only of sp3-bonded carbon, as with cubic diamond, and has the same density, but is thermodynamically metastable. However, a recent theoretical calculation suggests that hexagonal diamond has an indentation hardness higher by 58% than cubic diamond (Non Patent Literature 1). It has also been predicted that hexagonal diamond has higher Young's moduli than cubic diamond in most crystallographic orientations (i.e., resistant to deformation) (Non Patent Literature 2). Accordingly, hexagonal diamond has a promising potential as an industrial material, such as for ultrahard tools.